The present invention relates to a hub unit comprising an electric machine adapted for driving a wheel. The invention is further directed to a heavy vehicle and a work machine comprising such a hub unit.
The term heavy vehicle comprises different types of commercial transportation vehicles, such as trucks, buses and work machines. The term work machine comprises different types of material handling vehicles like construction machines, such as a wheel loader, an articulated hauler, a backhoe loader, a motor grader and an excavator. The invention will be described below in a case in which it is applied in a wheel loader. This is to be regarded only as an example of a preferred application.
A hub unit of a work machine normally comprises a so-called final drive, or hub-mounted reduction gear. The wheel is arranged rotationally rigidly on a hub and a planetary gear set is connected between a transverse drive shaft and the hub. The transverse drive shaft is driven by an angular gear, or center gear, which is in turn driven by an internal combustion engine (a diesel engine) by way of a transmission system. Arranging a planetary gear set on each drive wheel in this way produces a reduction in rotational speed from the transverse drive shaft to the hub and an increase in torque from the transverse drive shaft to the hub.
Increasing energy prices and reduced access to oil will in the future lead to an increasing demand in propelling vehicles by means of electricity. A vehicle comprising hub units provided with an electric machine at each wheel is known. The vehicle comprises an electric power generating means, which is connected to the electric machines in the hub units for supplying power to the electric machines. The electric power generating means may comprise an internal combustion engine (for example a diesel engine) and a generator for generating electric energy. By arranging an electric machine at each wheel, rotation of the wheels may be controlled individually. Further, a mechanical driveline connecting the internal combustion engine and the wheels may be dispensed with.
It is desirable to achieve a hub unit comprising an electric machine and a reduction gear, and which is adapted for a work machine operation. A further aim of the invention is to create conditions for a compact design in an axial direction in order to fit into a wheel hub.
According to an aspect of the present invention, the hub unit comprises at least two planetary gears coupled in series between the electric machine and a wheel hub, and a gear shifting device for connecting and disconnecting, respectively, one of said planetary gears from being drivingly connected to the electric machine.
The planetary gears are configured in order to adapt the input torque and the wheel speed. Each planetary gear preferably comprises one sun gear, one ring gear, one planet carrier and a plurality of planet wheels rotationally supported on the planet earner and in engagement with the sun gear and the ring gear.
By virtue of the gear shifting device, it is possible to shift gears between high and low gears. This is advantageous when using electric machines which have limited maximum rotation speeds, for example 4000 rpm. This maximum rotation speed would, according to a specific design of the planetary gear sets, correspond to a maximum speed of 10 km/h of the machine. Thanks to the gear shifting device, one of the planetary gear sets is disconnected and the maximum speed of the machine is substantially increased.
In other words, the gear shifting device is adapted to bypass one of said planetary gears in the power transmission path from the electric machine to the wheel hub when it is in the disconnected state.
Preferably, a sun gear forms an input to each planetary gear and a planet carrier forms an output from each planetary gear.
According to a preferred embodiment of the invention, a planetary gear member forms a planet carrier in an upstream planetary gear and a sun gear in a downstream planetary gear. Thus, the planet carrier in the upstream planetary gear and the sun gear in the downstream planetary gear rotates with equal speed. The planetary gear member may be formed in a one-piece unit or assembled by a plurality of parts.
According to a further preferred embodiment of the invention, the hub unit comprises three planetary gears coupled in series between the electric machine and the wheel hub. In this way, an output speed of the electric machine is reduced to a suitable wheel speed for the operation of the work machine.
According to a further preferred embodiment of the invention, an upstream planetary gear is arranged axially outwards of a downstream planetary gear. The wording “upstream” is defined referring to the power transmission path from the electric machine to the wheel hub. Preferably, the electric machine is arranged axially inwards of the series of planetary gears. Thus, the power transmission path extends from the electric machine axially outwards to the upstream planetary gear and then axially inwards to the downstream planetary gear. This configuration of the planetary gears creates conditions for a compact hub unit in the axial direction.
According to a further development of the last-mentioned embodiment, a planet carrier body in a last planetary gear in the series comprises a portion that extends radially outside an upstream planetary gear and is adapted to be rotationally rigidly connected to the wheel hub. Such a configuration creates conditions for an even more compact hub unit in the axial direction. Preferably, for the embodiment with three planetary gears in series, a planet carrier in each of a first planetary gear and a second planetary gear in the series is positioned so that a planet wheel supporting pin extends from a planet carrier body in an axially outwards direction, and that a planet carrier in a third planetary gear in the series is positioned so that a planet wheel supporting pin extends from a planet carrier body in an axially inwards direction. This configuration is especially advantageous with regard to being compact in the axial direction.
Further preferred embodiments will be apparent from the following description and drawings.